FIG. 1 is a side cross-sectional view showing the basic construction of a wet-type multiplate clutch 10. The drawing shows a clutch case 1, separator plates 2, friction plates 3, and a piston 4 for pressing the separator plates 2 and friction plates 3 toward a stopper ring 5. When pressure oil is introduced into an oil compartment 41 through an oil hole 44, the piston 4 is pressed rightwards as viewed in the drawing. When the pressure oil is drawn, on the other hand, the piston 4 is returned leftwards by a return spring 43 arranged between a piston support 42 and the piston 4. At this time, pressure oil is allowed to enter the oil compartment 41 through an oil hole 45 under centrifugal force, and acts to press the piston 4 back leftwards. Designated at numeral 46 is a stopper ring for the piston support 42.
The separator plates 2 are maintained at outer peripheral splines 25 thereof in fitting engagement with spline grooves 11 of the clutch case 1. Numeral 12 indicates inner peripheral splines of the clutch case 1, and these inner peripheral splines are maintained in fitting engagement with a center shaft (not shown). In the drawing, an alternate long and short dash line X—X indicates a central axis.
FIG. 2 is a front view of the separator plate 2. Numeral 20 indicates a friction surface, and numeral 25 designates the splines formed on the outer periphery.
A wet-type multiplate clutch making use of such conventional separator plates is unavoidably accompanied by an inconvenience that its friction characteristics vary considerably depending on the oil temperature. Especially when the oil temperature is low and the engagement load is also low, a problem arises in that, as will be mentioned subsequently herein, oil films are formed between the separator plates and their associated friction plates to extremely lower the coefficient of friction.
As disclosed in JP 6-337023 A, it was hence proposed to roughen the surface of a separator plate by brushing to form oil grooves between lands, and then to round peak portions of the lands.
The above-described conventional separator plate is, however, still accompanied by the drawback that the peak portions of the lands, said peak portions being to project toward a friction material as its counterpart member when the separator plate is assembled in a wet-type multiplate clutch, unavoidably causes the friction material to wear out, although the peak portions of the lands with oil grooves formed therebetween have been rounded.